[ussuri][goal] Drop python 2.7 support and testing

OpenStack is dropping the py2.7 support in ussuri cycle.

ironic-tempest-plugin is ready with python 3 and ok to drop the
python 2.7 support.

All stable jobs need to run on py3 as master ironic-tempest-plugin
cannot support py2 anymore. Any py2 cloud/env can be tested by
installing the Temepest and plugins on py3 venv.

Complete discussion & schedule can be found in
- http://lists.openstack.org/pipermail/openstack-discuss/2019-October/010142.html
- https://etherpad.openstack.org/p/drop-python2-support

Ussuri Communtiy-wide goal:
https://governance.openstack.org/tc/goals/selected/ussuri/drop-py27.html

Change-Id: Ibea78605aedb955e3dc70252ca380c7e5f8f8726
diff --git a/zuul.d/stable-jobs.yaml b/zuul.d/stable-jobs.yaml
index 759239a..ec95d55 100644
--- a/zuul.d/stable-jobs.yaml
+++ b/zuul.d/stable-jobs.yaml
@@ -4,7 +4,7 @@
     override-checkout: stable/train
     vars:
       devstack_localrc:
-        USE_PYTHON3: False
+        USE_PYTHON3: True
 
 - job:
     name: ironic-standalone-stein
@@ -12,7 +12,7 @@
     override-checkout: stable/stein
     vars:
       devstack_localrc:
-        USE_PYTHON3: False
+        USE_PYTHON3: True
 
 - job:
     name: ironic-tempest-functional-python3-train
@@ -31,7 +31,7 @@
     override-checkout: stable/train
     vars:
       devstack_localrc:
-        USE_PYTHON3: False
+        USE_PYTHON3: True
 
 - job:
     name: ironic-tempest-ipa-wholedisk-direct-tinyipa-multinode-stein
@@ -39,7 +39,7 @@
     override-checkout: stable/stein
     vars:
       devstack_localrc:
-        USE_PYTHON3: False
+        USE_PYTHON3: True
 
 - job:
     name: ironic-inspector-tempest-train
@@ -49,7 +49,7 @@
       devstack_localrc:
         FIXED_NETWORK_SIZE: 4096
         EBTABLES_RACE_FIX: True
-        USE_PYTHON3: False
+        USE_PYTHON3: True
 
 - job:
     name: ironic-inspector-tempest-stein
@@ -59,7 +59,7 @@
       devstack_localrc:
         FIXED_NETWORK_SIZE: 4096
         EBTABLES_RACE_FIX: True
-        USE_PYTHON3: False
+        USE_PYTHON3: True
 
 - job:
     name: ironic-inspector-tempest-discovery-train
@@ -67,7 +67,7 @@
     override-checkout: stable/train
     vars:
       devstack_localrc:
-        USE_PYTHON3: False
+        USE_PYTHON3: True
 
 - job:
     name: ironic-inspector-tempest-discovery-stein
@@ -75,4 +75,4 @@
     override-checkout: stable/stein
     vars:
       devstack_localrc:
-        USE_PYTHON3: False
+        USE_PYTHON3: True